The invention relates to a process involved in the manufacture of thick, preferably sheet-shaped, thermoplastic molded parts or molded parts having areas of variable cross sections, wherein at least one part of a plastic material is to be conveyed as a melt or molten material into a heatable and optionally coolable mold, e.g. a compression mold or an injection mold, and therein is compressed or injection molded.
Such a process is of interest especially for the manufacture of a chamber and diaphragm filter plate which includes a thick-walled edge and a significantly thinner inner area. This shape of chamber and diaphragm filter plates is necessary for use in so-called filter presses, since the filter chambers of such filter presses are formed by depressions in the individual filter plates when the plates are clamped together, arranged in a row with their edges lying side-by-side, into a mold plate assembly. In so doing, the significantly thicker edges of the plates serves as a sealing surface to prevent suspensions, which are pressed into the filter chamber and are to be filtered, from escaping.
The manufacture of such thick molded parts, especially those having variable thickness, causes problems because, while cooling in the mold following the manufacture from molten plastic, voids, sink marks in the surface and stresses that can lead even to crack formation form due to the large, especially varying material thickness. Therefore, cooling in the compression mold must be conducted very slowly and carefully under constant molding pressure, a process that is not only very time consuming, but also that requires that the compression mold be occupied for such period of time.
To manufacture such molded parts of variable cross section with largely homogeneous material joints without the formation of voids, a process a proposed in DE 31 38 858 C3 in which a preform is inserted as a two-sided flat plate into the open compression mold. Then the compression mold is closed and the preform is forced to close under heat and pressure, wherein in a first phase little pressure is applied to the preform in order to melt the preform on both sides by means of the heated compression mold. Then in a second phase that is temporally adjusted to the melting process of the surface of the preform, the distance between upper and bottom parts of the compression mold is increasingly reduced by increasing the pressure. The material melted from both surfaces of the sheet-shaped preform is forced into peripheral areas by means of the pressure of the compression mold and finally fills out the entire peripheral area. This type of manufacture requires much time and technical experience and does not exclude with certainty the formation of voids, sink marks in the surface of the finished molded part, and stresses that form and can lead to the formation of cracks.
A process known from DE 31 38 857 has the same goal of manufacturing molded parts with homogeneous material joints without the formation of voids. In this process, a pre-finished partial molding, namely a frame forming a finished edge, and supporting cams in a plate area, are inserted into the compression mold. The cams are intended for the mutual support of the filter plates in the mold plate assembly. Subsequently, other cavities remaining in the compression mold are filled with plastic granules and then compressed with the pre-finished frame forming the edge and the supporting cams while the compression mold is heated. With this process the finished molded part is supported to shrink everywhere by the same amount when cooling. Apart from the fact that the process is relatively time consuming and expensive, stresses and fractures, especially in the area of transition from the plate to the supporting cam and to the peripheral area, cannot be ruled out with certainty.